Computerized systems provide many advantages towards peoples' ability to perform tasks. Indeed, the computer systems ability to process information has transformed the way we live and work. Computing systems now take a wide verity of forms including desktop computers, laptop computers, tablet PCs, Personal Digital Assistances (PDAs), and the like. Even household devices (such as refrigerators, ovens, sewing machines, security systems, and the like) have varying levels of processing capability, and thus may be computing systems. Processing capabilities continue to be incorporated into devices that traditionally did not have such processing power. Accordingly, the adversity trend of computing systems will likely increase.
Along with computing systems, the Internet has revolutionized the way people communicate and has ushered in a new era in human history often termed the “information age.” In essence, the Internet includes a large constellation of networked computers that are spread out over much of the world. Sophisticated computers, software, and networking technology have made communication over the Internet fairly straight forward from the view point of the end user. In order to communicate over a network, typically one computing system constructs or otherwise accesses an electronic message and transmits the electronic message over a network to another computing system. Portions of the electronic message may be read by human users as when the electronic message is an email, instant message, an appointment, or the like. Alternatively, or in conjunction, the electronic message may be read by an application running on the receiving computing system. The electronic message may further be constructed by an application running on the sending computing system with the possible assistance of a human user.
In some environments, applications communicate with one another using queued message communication. Queued communication includes mechanisms for a sending application to write a message into a sending queue, the sending queue to transfer the message to a receiving queue, and for a receiving application to read the message from the receiving queue. The queues maintain communication state outside of the communicating parties, and provide a level of indirection between them. Accordingly, queued messaging provides reliable communication between loosely coupled applications. Senders and receivers of messages use intermediary queue managers to communicate, and may independently shut down and restart and may even have non-overlapping lifetimes. Queuing also allows clients and servers to send and receive messages “at their own pace” with the queue taking up the slack at either end.
One model of a queued messaging system is a published/subscribed (often termed pub-sub), which is an event driven architecture with the notions of subscriptions, events, and notifications. A subscription is a request made by a consumer or registrar to a data store, expressing interest in some change thereto (typically by specifying a scope and a criterion) that are expected to happen in the data store. Events, on the other hand, are actual changes that happen in the data store. When an event happens in a data store for which interest was expressed by a subscription, a notification is published and subsequently sent to a registrar of that subscription notifying them of the event.
There are many examples of such pub-sub uses. For example, a user with online access that has previous booked or otherwise scheduled travel arrangements such as flight plans, hotel accommodations, car rentals, and meeting schedules may wish to be notified of any changes in his travel plans. Accordingly, a registrar can be developed for a subscription that periodically polls one or more servers with knowledge of the user's travel plan events. The registrar may then inform the user of any changes identified.
Another example of where subscriptions are useful is in an email type environment where a user wishes to be notified of certain events and/or assigns an assistant to act on behalf of such events. For example, the user or an assistant may wish to know of events such as: when appointments have been scheduled and/or changed; the arrival of urgent email messages; when messages or folders have been created, deleted, updated, etc.; activities that happen on specific mailboxes; and other various events. Upon notification, the user or assistant may take action as appropriate.
Although pub-sub systems provide a convenient way of notifying users and registrars of events, existing implementations do not perform well under the following two conditions: a large number of subscriptions; and/or a very high event rate. Typically, all of the events are stored in an events table and all of the subscriptions are stored in a subscription table. Each event within the events table is then evaluated against each subscription within the subscription table. The results of each comparison are then stored in a separate table. As can easily be seen, because each event is evaluated against each subscription, and the results thereof stored, a high event rate and/or high number of subscriptions can create a tremendous processing burden and otherwise consume valuable system resources (e.g., memory).